1. Field of the Invention.
This invention relates to improved cryptographic techniques for public-key digital signatures, and more specifically to such signatures achieving blind signatures and one-show signatures.
2. Description of Prior Art.
Known public key digital signatures based on RSA fix, at least for a large class of messages, the choice of public exponent. One problem with these systems is that they must employ a second cryptographic transformation, sometimes called a "hash function." Consequently, their security is forced to rely at least on the hash function, and thus not only on the difficulty of finding roots in the RSA system.
A somewhat more demanding use of digital signatures was disclosed in U.S. Pat. No. 4,759,063, titled "Blind Signature Systems," issued to the present applicant, also appearing as European Patent Publication No. 0139313 dated 2/5/85, and which is incorporated herein by reference. Such blind signatures were later extended, as described in the co-pending application of the present applicant, titled "One-Show Blind Signature Systems," filed 3/3/88, with U.S. Ser. No. 168,802, now abandoned, and also incorporated herein by reference. Still other related signatures were described in a co-pending application of the present applicant, titled "Unpredictable Blind Signature Systems," with U.S. filing date 5/4/89, U.S. Ser. No. 347,303, also incorporated herein by reference.
These one-show systems appear to have rather wide utility, but the amount of data which must be stored can be reduced, as can the amount of data communicated. The party showing such a signature might wish protection against false charges that the signature has improperly been shown more than once, particularly in the case where the accuser has substantial computing capabilities.